1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to designing methods for dimple patterns of golf balls.
2. Description of the Related Art
Golf balls have a large number of dimples on the surface thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. By causing the turbulent flow separation, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulent flow separation promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golfball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. The reduction of drag and the enhancement of lift force are referred to as a “dimple effect”.
Generally, a golf ball is formed by a mold having upper and lower mold halves. Since a molding material (e.g. a synthetic resin) leaks out from the parting face between the upper and lower mold halves, a flash is generated along the equator portion on the surface of the golf ball. The flash is generated along the parting line. The flash is ground and removed with a whetstone or the like. Removal of a flash generated inside the dimple is difficult. In order to facilitate the removal of the flash, no dimple is formed on the equator. In other words, no dimple is provided on the parting face of the mold. A great circle path is formed on the seam of a golf ball obtained by using this mold. The great circle path agrees with the equator. When a flash is removed, the land near the equator may be removed together with the flash. Due to this removal, dimples are deformed. In addition, the dimples near the equator tend to be orderly arranged.
Thus, the equator has the following characteristics:
(1) a great circle path exists;
(2) the dimples near the equator may be deformed; and
(3) the dimples near the equator tend to be orderly arranged.
On the surface of the golf ball, the region near the equator is a unique region.
Rotation in which the rotational axis of backspin passes through both poles is referred to as PH rotation. Meanwhile, rotation of which the rotational axis is orthogonal to the rotational axis of PH rotation is referred to as POP rotation. As described above, the region near the equator is a unique region. During PH rotation, a part where the greatest circumferential speed of the backspin is attained agrees with the equator. Thus, a sufficient dimple effect is not obtained. The dimple effect during PH rotation is less than the dimple effect during POP rotation. The difference between these dimple effects impairs the aerodynamic symmetry of a golf ball. Other than a shot at a teeing ground, golf players cannot decide a hitting place of a golf ball. Thus, the flight distance of a golf ball with inferior aerodynamic symmetry is variable. Golf players have difficulty in landing this golf ball at an intended point.
The United States Golf Association (USGA) has established the rules about aerodynamic symmetries of golf balls. A golf ball having a large difference between the trajectory during PH rotation and the trajectory during POP rotation does not conform to the rules. The golf ball cannot be used in golf tournaments.
JP-S61-284264 discloses a golf ball in which the dimples near the seam are greater in volume than the dimples near the poles. This volume difference contributes to eliminating the aerodynamic asymmetry caused by the uniqueness of the region near the equator. A similar golf ball is also disclosed in JP2000-93556.
JP-H9-164223 discloses a method in which a large number of dimples are randomly arranged by a computer. A golf ball having randomly arranged dimples has excellent aerodynamic symmetry. A similar method is also disclosed in JP2000-189542.
The golf ball disclosed in JP-S61-284264 eliminates, by the volume difference of dimples, the disadvantage caused by the dimple pattern. The disadvantage caused by the dimple pattern is eliminated not by modification of the dimple pattern. In the golf ball, the potential of the dimple pattern is sacrificed. The flight distance of the golf ball is insufficient. Similarly, the flight distance of the golf ball disclosed in JP2000-93556 is also insufficient.
In the method disclosed in JP-H9-164223, trials and errors are required for deciding a pattern, and it takes a long period of time for this decision. In this method, since the pattern depends on the computer, an intention of a designer is unlikely to be reflected in the pattern. In addition, the pattern obtained by this method has a wide variety of dimples. Thus, time and labor are required for producing a mold. The method disclosed in JP2000-189542 also has similar problems.
An object of the present invention is to provide a designing method by which a pattern having excellent aerodynamic symmetry can be easily obtained.